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so I am wondering other members thoughts and experiences on the concept of a composite regenerator.

What I mean is a regenerator comprised of different materials that are either interwoven or alternately layered.

Such a matrix could have the benefit of being able to tune the regenerator response, relative to material heat capacity, density and also different rates of material conduction.

I'll give a 'bad' example of what I mean by tuning by referencing a supercritical fluid thread from this forum, (remember what I’m talking about has nothing to do with supercritical fluids, per se) :
although very different to the concept of a composite regenerator, this demonstrates the concept of 'tuning' and the regenerator idea is partly spawned from the following thread Stirling Engines with Supercritical Fluids? and the known response of different supercritical fluids properties to mix together in a composite form and interact in a known and predictable manner.
i'll refer to this extract from its related wiki page .
wiki...
All supercritical fluids are completely miscible with each other so for a mixture a single phase can be guaranteed if the critical point of the mixture is exceeded. The critical point of a binary mixture can be estimated as the arithmetic mean of the critical temperatures and pressures of the two components,
http://en.wikipedia.org/wiki/Supercritical_fluid 


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I propose that a blended/interwoven regenerator may behave as an average of the combined material properties and be able to be mixed in different proportions and 'tuned' to improve regenerator thermal behavior in accordance to different engine needs.

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Another approach to a composite regenerator would be using a 'layered method' of a variety of different conductors.
By doing this, one may be able to focus different rates of conduction of materials for different parts of the regenerator. e.g. hot end, middle section and cold end of a regenerator may require different thermal properties. Its probably a pretty dynamic variation from one end to the other but I suspect/propose that a regenerator with variable thermal properties along its length 'might' improve its overall regenerative function and therefore engine output and efficiency.

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a third approach could be a combination of the previous 2 proposed ideas, but with different blends and proportions of different materials that taper in and out along the length of the regenerator

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I realise there could be problems such as induced galvanic corrosion, depending on different materials interaction with each other and their relative locations in the electrochemical series (although in a sealed system with an inert working gas this would not be an issue).
I have no immediate intention of undertaking such investigations but thought I’d put it out there, to see if anyone else has been down this path.

vamoose

Vamoose,
It really sounds like an interesting area of investigation. Do you have any theories of composition ? The layering would be the easiest to construct. Your thinking of things like steel wool, ss wool and copper? You might even consider a core of one material and an outer blanket of another. Even different size mesh on each material would increase or decrease air flow. A single layer of a nonconductor mesh between , could eliminate electrolysis .

I would suggest that you build the motor so that the regenerator section can be opened easily, then start with a standard, so stainless steel foil, and test the power out put with a generator of some sort is the easy way. Then change the regenerator, and retest.
One suggestion for a regenerator might be, top 1/3 fine copper wire, middle 1/3 stainless steel wool, bottom 1/3 either more copper, or aluminium. I suspect that the stainless foil will be the best, but it's worth trying, all sorts of materials have been tried in the past. Ian S C

To complement Ian's suggestion, all metallic layers could be made of foil. You could randomly dimple both sides of each sheet for better airflow. A thin layer of carbonized paper could be used to separate differing metals. Iam not sure I understand the working hypothesis. Do you want the top and bottom to absorb and release heat rapidly while the middle stores and releases heat slowly?

I'm sure you understand it correctly foha.
The idea is pretty unrefined on my part, and I like the suggestions so far.
If you start to look at it, there are a number of different approaches that could be applied.
just a loose suggestion is maybe materials that are less conductive should be at the cold end (as it is closer to Zero kelvin) and materials slightly more conductive at the hot end (further from zero kelvin), this might help maintain a more consistent rate of heat exchange along the length of the regenerator (if this is even what we are after for best results).

Maybe we could just increase/decrease the surface area to mass ratio of a single material along the length..ie increasingly coarser strands (say stainless steel), or change the amount of material progressively along the length with the same coarseness.
I guess using a 'single' material with other variables would make it a different class of 'composite' regenerator though.

vamoose

There is a simple way of dimpling the foil. Go to a shop selling sewing equipment (pins, needles, thread etc.), and buy a tracing wheel, this little tool consists of a handle and a little star shaped wheel, just roll the wheel across the foil. This method has been used by a number of published Stirling Engine builders. Ian S C

What if you could increase or decrease the working gas flow through the displacer / regenerator at will ? Not only control its flow but also where that flow takes place in the regenerator. What Iam thinking of is sieve plates that progressively line up. Stepped magnets, on the outside would pull the plate it lines up with. Each plate would have a small magnet on it. You could open or close any plate for experimentation, just by moving the magnets on the outside of the cylinder.

fullofhotair wrote:What if you could increase or decrease the working gas flow through the displacer / regenerator at will ? Not only control its flow but also where that flow takes place in the regenerator. What Iam thinking of is sieve plates that progressively line up. Stepped magnets, on the outside would pull the plate it lines up with. Each plate would have a small magnet on it. You could open or close any plate for experimentation, just by moving the magnets on the outside of the cylinder.

This, my friend, is a very very good example of thinking. Inside, outside or without a box, that idea is top shelf!

Add induction to your idea. The displacer could be heated on one end via a pulse of magnetism focused on when the cyclic phase brought it closest to the hot end. The outside application of energy to hot end might be eliminated. IE the "flame" could go away if the heat was applied only when it was needed, and internally.

What about applying energy outside the regular EM band we normally think of. What about inducing the heat with microwaves? A magnetron focused on the end of the displacer, timed, so that it heats only when, and what, as needed. Really low wasted energy?

See what happens when you come up with a good idea! This is the way shared thinking works!

R

Theropod,
Would an old dismantled microwave oven work for that application? Would you have to shield it in some way? Do you think it would be just a short burst or a longer pause of released microwave? The heat would be exactly where you want it. The cylinder could be a ceramic cup. Could you use the thermoelectric effect on the cold side as a feed back loop?

silcon nitride, would be a good ceramic to use for the cylinder. It is RF transparent, extreme thermal shock resistant, high toughness The microwave could be focused beyond the bottom of the hot side ,to just within the cylinder to an absorber. No heat would be lost from the hot end.

Where does the power for the magnetron come from, that will be around 1Kw, you might as well use that for an electric motor. Ian S C

The magnetron would only have to fire for a few degrees of the dwell time while the displacer "hot" end is near BDC. So that 1K wouldn't have to be constant.

Yes, I think some serious shielding would be needed, maybe encasing the entire engine.

R